The present invention relates to the recirculation of exhaust gas in an internal combustion engine and particularly for purposes of reducing harmful emissions from the engine exhaust. In recent years exhaust gas recirculation (EGR) has been controlled in engines employed for automotive vehicles by providing a crossover passage within the engine intake manifold for inter-connecting the exhaust passages with the combustion chamber induction passages. Typically, a valve seat is provided in the passage with a movable poppet valve for controlling flow over the valve seat from the engine exhaust manifold passages into the combustion chamber inlet passages. Opening of the poppet valve is controlled to permit greater or lesser EGR flow depending upon the engine operating conditions and in particular the engine load.
Known systems for controlling the opening of the EGR valve in an engine have employed a pressure responsive diaphragm connected to the poppet valve by means of a guided and insulated operating rod extending externally of the engine intake manifold passage. In certain known EGR control systems, the diaphragm is moved by a fluid pressure signal which may be provided from a pressure tap on the engine exhaust, or from a vacuum signal generated from the engine throttle induction passage or from a vaccum signal controller employing the engine exhaust gas to modify the induction passage vacuum to provide a control signal. Various strategies have been developed to provide the control signal to the pressure responsive diaphragm thereby providing the desired program for EGR flow over the various combinations of engine speed and load experienced in service.
For engines requiring a wide variation of EGR flow and therefor a complicated program control strategy over the range of operating conditions expected in engine service, it has proven costly and difficult to provide a suitable vacuum control signal for acting on the valve power diaphragm because of the opposite nature and wide variation in the available induction vacuum and exhaust back-pressure as generators for the control signal. Furthermore; the trend in modern engine design has been directed away from suction-venturi carburetion and toward throttle-body or multi-point intake port fuel injection in which the injectors are electrically operated. The advent of microprocessors has enabled engine controls to be operated with sophisticated control signals generated by the microprocessor from numerous electrical transducer inputs as for example engine speed, temperature, exhaust gas concentration, intake manifold depression and ambient air temperature. Once a microprocessor has been incorporated in the control system of an engine, it is relatively easy and inexpensive to provide an additional control signal from the microprocessor for providing the desired program strategy for EGR.
Therefore, it has been desired to find a way or means for providing an electrically controllable EGR valve which can operate from a low voltage signal, as for example from a microprocessor, and provide rapid response to changing engine operating conditions and yet provide to a degree of accuracy the desired EGR flow for the sensed engine operating conditions.
Heretofor, it has been proposed to provide a direct current servomotor, operable through a speed reducer to move the EGR poppet operating rod. In such an arrangement it is known to provide a variable potentiometer movable with the poppet operating rod to provide a feedback signal of poppet position for terminating energization of the servomotor. However, this type of arrangement for moving the EGR valve poppet has been found costly to manufacture and difficult to maintain in engine compartment operating environments with a desirable degree of accuracy because of elevated temperatures and contamination from oil and dirt. Furthermore, it has been found difficult to provide a DC servomotor drive for an EGR valve with a desired degree of accuracy where the servomotor drive is exposed to the vibration regime encountered in a typical automotive engine application. In particular, where a feedback potentiometer is employed to control servomotor operation, it has proven to be quite costly to provide a potentiometer capable of giving the desired position accuracy and also capable of withstanding the rigors of the engine compartment operating environment. Thus it has long been desired to provide a way or means of precisely controlling the movement of the EGR poppet valve in an automotive engine application by means of an electrical control signal with a high degree of reliability and low manufacturing cost.